Liquid drop modulators have been developed for ink jet recording or printing. Ink jet printing systems offer numerous benefits, including extremely quiet operation when printing, high speed printing, a high degree of freedom in ink selection, and the ability to use low-cost plain paper. The so-called “drop-on-demand” drive method, where ink is output only when required for printing, is now the conventional approach. The drop-on-demand drive method makes it unnecessary to recover ink not needed for printing.
Liquid drop modulators for ink jet printing include one or more nozzles which allow the formation and control of small ink droplets to permit high resolution, resulting in the ability to print sharper characters with improved tonal resolution. In particular, drop-on-demand ink jet print heads are generally used for high resolution printers.
Drop-on-demand technology generally uses some type of pulse generator to form and eject drops. For example, in one type of print head, a chamber having an ink nozzle may be fitted with a piezoelectric wall that is deformed when a voltage is applied. As a result of the deformation, the fluid is forced out of the nozzle orifice as a drop. The drop then impinges directly on an associated printing surface. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies when printing directly to paper or alternative substrates. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
Another type of print head uses bubbles formed by heat pulses to force fluid out of the nozzle. The drops are separated from the ink supply when the bubbles collapse. This type of droplet ejection system based upon thermally generated bubbles, commonly referred to as the “thermal ink jet” or “bubble jet” system, has high speed printing capability.
Yet another type of drop-on-demand print head incorporates an electrostatic actuator. This type of print head utilizes electrostatic force to eject the ink. Examples of such electrostatic print heads are disclosed in U.S. Pat. No. 4,520,375 to Kroll and Japanese Laid-Open Patent Publication No. 289351/90. In particular, U.S. Pat. No. 6,367,915 to Gooray et al. discloses electrostatically or magnetically driven piston structures whose movement ejects a drop or droplet of fluid.
While there are many ink compositions known to be suitable for use with piezoelectric devices or bubble jet device, a need remains for ink compositions suitable for use with electrostatic liquid drop modulator devices such as those disclosed in U.S. Pat. No. 6,367,915 to Gooray et al. Electrostatic liquid drop modulation requires a fluid that possesses very specific dielectric properties. For electrostatic actuation, the maximum force is realized at a maximum product of field and dielectric constant. High field requires a high dielectric strength (high breakdown voltage) in the fluid. The fluid must exhibit a sufficiently low electrolytic decomposition rate to support a stable field. A fluid must also possess a low conductivity to minimize power losses due to the migration of ions to the electrodes and maximize energy efficiency of the device. The use of non-ideal conductors raises the possibility of capacitively coupling of the field to solution and the need for low conductivity in the fluid to ensure sufficiently long field relaxation times. For neat fluids, conductivity generally scales with dielectric constant, demonstrating that simple solvent selection is not enough to guarantee a fluid with optimum properties. Additions of solvent and solute impact both the dielectric constant and the conductivity. Appreciable solvent and solute create a media of mixed dipoles, reduces hydrogen bonding in water and may generate a greater concentration of mobile ion, often resulting in decreases in dielectric constant.
Specifically, the piston driven microelectromechanical system (MEMS) based liquid drop ejector technology requires ink with special electrical properties. The ink needs to satisfy the low conductivity of less than 200 microS/cm and high dielectric constant greater than 60. Purified deionized water can satisfy the requirements. However, the addition of water soluble dye to the deionized water will increase the conductivity of the ink to well above 200 microS/cm. An oil based ink (or hydrocarbon based ink) will have low conductivity below 200 microS/cm and low dielectric constant below 10. The addition of colorants to the oil, e.g. oil soluble dyes, can satisfy the low conductivity requirements, but cannot satisfy the high dielectric constant requirements. Therefore, aqueous inks with water soluble dyes or oil based ink with oil soluble dyes cannot satisfy the piston-MEMS ink requirements.
The present invention is directed to overcoming these deficiencies in the art.